In the semiconductor industry, critical steps in the production of integrated circuits are the selective formation and removal of films on an underlying substrate. Typical processing steps involve (1) depositing a film; (2) patterning areas of the film using lithography and etching; (3) depositing a film which fills the etched areas; and (4) planarizing the structure by etching or chemical-mechanical polishing (CMP).
In film removal processes, it is extremely important to stop the process when the correct film thickness has been removed (that is, when the endpoint has been reached). In a typical CMP process, a film is selectively removed from a semiconductor wafer by rotating the wafer against a polishing pad (or moving the pad against the wafer, or both) with a controlled amount of pressure in the presence of a slurry. Overpolishing (removing too much) of a film renders the wafer unusable for further processing, thereby resulting in yield loss. Underpolishing (removing too little) of the film requires that the CMP process be repeated, which is tedious and costly. Underpolishing may sometimes go unnoticed, which also results in yield loss.
In a number of CMP processes, it is necessary to measure the thickness of the layer to be removed and the polishing rate for each wafer, in order to determine a desired polishing time. The CMP process is simply run for this length of time, and then stopped. Since many different factors influence the polishing rate, and the polishing rate itself can change during a process, this approach is far from satisfactory.
A number of other methods have been suggested for obtaining reliable endpoint detection in CMP processing. In general, these methods each have inherent disadvantages, such as a lack of sensitivity, an inability to provide real-time monitoring, applicability to only certain types of films, or requiring removal of the wafer from the process apparatus to test for endpoint.
U.S. Pat. No. 5,559,428 to Li et al. describes an in-situ endpoint detection scheme for conductive films, using an induction method. There remains a need for an in-situ, real-time endpoint detection scheme suitable for use with nonconductive films. Such a scheme should also have high detection sensitivity and fast response time. In addition, it is desirable that the detection apparatus be robust, inexpensive and require little maintenance.
One important CMP process involves removal of a polycrystalline silicon (poly-Si) film overlying a patterned film of silicon dioxide (SiO.sub.2) or silicon nitride (Si.sub.3 N.sub.4); after removal of a blanket layer of poly-Si, a surface having partly poly-Si and partly SiO.sub.2 or Si.sub.3 N.sub.4 will be exposed. FIG. 1 shows a typical CMP apparatus 10 in which a workpiece 100 (such as a silicon wafer) is held face down by a wafer carrier 11 and polished using a polishing pad 12 located on a polishing table 13; the workpiece is in contact with slurry 14. The wafer carrier 11 is rotated by a shaft 15 driven by a motor 16. 2A is a detail view showing a patterned oxide layer 102 with an overlying layer 104 of poly-Si. Generally, it is necessary to remove the target film of poly-Si down to a level 105 so as to completely expose the oxide pattern, while leaving the oxide layer itself essentially intact (see FIG. 2B). Accordingly, a successful endpoint detection scheme must detect exposure of the oxide layer with very high sensitivity, and automatically stop the CMP process within a few seconds after the oxide becomes exposed (that is, no operator intervention should be required when endpoint is reached). Furthermore, the endpoint detection scheme should be effective regardless of the pattern factor of the wafer (that is, even if the area of the exposed underlying oxide layer is a small portion of the total wafer area).
One widely used approach to monitor and control a CMP process is to monitor a change in the motor current associated with a change in friction between (a) the top surface of the polishing pad 12 and (b) the slurry 14 and the surface being polished (such as the surface of wafer 100). This method is satisfactory when there is a significant change in friction as the underlying layer is exposed. However, for many applications, including the poly-Si polishing process described just above, the change in friction associated with the interface between layers is too small to result in a motor current change sufficient to be a reliable indicator of CMP process endpoint. This problem is aggravated by a large noise component in the motor current associated with the typical feedback servo current used to drive the wafer carrier at a constant rotational speed. In addition, a small pattern factor (that is, a relatively small area of the underlying patterned layer, compared with the area of the target layer) causes only a small change in friction as the endpoint is reached, limiting the useful signal.
When the motor current approach is used, an adequate signal-to-noise ratio may sometimes be obtained by varying process parameters (such as downward pressure on the polishing pad and relative rotational speed of the table and wafer carrier). Accordingly, optimization of process parameters for endpoint detection compromises other aspects of the CMP process, thereby compromising the product wafer quality.